All-optical cryptography through metasurface based on phase changeable nanoantenna

Nanophotonic all-optical devices have been enthusiastically studied to overcome the speed bottleneck of electrical devices. Recently, chalcogenide phase change material Ge2Sb2Te5 (GST) based active metasurfaces have emerged as a novel platform as complement of all-optical device. GST has two phase, so called amorphous and crystalline phases that can be changed within nanosecond scale by applying external thermal stimuli. Hence, various ultrafast 2-level switchable GST based meta-devices have been proposed and demonstrated. However, since only two switching levels are provided, there are limitations in applying to various photonic fields. In this paper, we expand its switching functionality to three-level through designing two different sizes of GST nanorod that exhibits different heat generation density in case of top-hat shape light illumination. Thanks to this thermo-optical phenomenon, we can obtain partial phase changed state, so called intermediate state once the appropriate intensity of control light is illuminated. Harnessing intermediate state, we propose novel cryptography platform that can provide high security level encryption while providing ultrafast communication speed.